1. Field of the Invention
The present invention relates to a color Doppler ultrasonic diagnostic apparatus which visualizes a blood flow utilizing Doppler signals acquired by transmitting/receiving ultrasonic waves, and in particular to an ultrasonic diagnostic apparatus having a function of reducing motion artifacts occurring due to changes in a speckle pattern that correspond to a motion of a tissue.
2. Description of the Related Art
In an ultrasonic color Doppler method, ultrasonic waves are transmitted/received to/from an object, and the Doppler signal that corresponds to the blood flow, which is a component of an ultrasonic echo signal occurring in the object, is two-dimensionally displayed. The ultrasonic echo signal acquired from the object is a convoluted signal, in which the signal from the tissue is convoluted with a blood flow signal that is the Doppler signal that results from the blood flow. Accordingly, an MIT (moving target indication) filter (which is also referred to as a “wall filter”) is used which is based on the fact that the motion velocity of the tissue is slow, thereby suppressing a clutter signal which is the Doppler signal from the tissue.
However, in a case in which there is a large amount of tissue motion due to breathing or a beating of the heart, an amplitude of the speckle pattern changes. Here, the speckle pattern is a granular pattern occurring due to a random interference of ultrasonic waves. Accordingly, the clutter signal cannot be sufficiently suppressed using only the MTI filter having a function of a suppressing signal under fixed conditions. As a result, such an arrangement has a problem of color artifacts which are referred to as “motion artifacts”.
In order to reduce the motion artifacts, a method has been proposed, in which the motion velocity of the tissue (clutter velocity) is calculated, and a phase correction is performed for the Doppler signal of an original signal, or properties of the MTI filters are adjusted (for example, see Japanese Unexamined Patent Application publication No. 10-99333, Japanese Unexamined Patent Application publication No. 11-267125, U.S. Pat. No. 5,349,524 specification).
The aforementioned method in which the clutter velocity is calculated and phase correction is performed for the original signal is an effective method without adverse effects as long as the phase correction is performed for the Doppler signal from a heart or a large blood vessel. However, the motion artifacts cannot be sufficiently suppressed by only the phase correction performed for the Doppler signal. In order to solve this problem, a technique has been proposed in which additional adjustments are made to a cut-off frequency of the MTI filters.
Also, a technique has been proposed in which processing is performed utilizing the baseband (IQ) signal generated in a reception circuit of the ultrasonic diagnostic apparatus, and the amplitude thereof, which is equivalent processing to the instantaneous clutter velocity correction processing (for example, see Japanese Unexamined Patent Application publication No. 2005-312773).
With the aforementioned conventional processing, the cut-off frequency is increased according to an increase in the clutter velocity. However, in some cases, such conventional processing with settings that are sufficient to ensure that the motion artifacts are suppressed has a problem that the blood flow signal is removed. Accordingly, such processing for adjusting the cut-off frequency of the MTI filters is not actively employed. Accordingly, in this situation, it is difficult for the ultrasonic color Doppler method to sufficiently remove the motion artifacts occurring due to the tissue motion.